Radio Broadcast (Nov 1926-Apr 1927)

Equipment for the Home-Constractor Some of the New E«J >y the By THE LABORATORY STAFF THE first articles of new equipment to be described this month are the chokes, impedances, and transformers made by the Samson Electric Manufacturing Company, and which are illustrated on this page. These various pieces of apparatus were tested in the Laboratory and some data are given below with regard to their characteristics and uses: Type 85 — This is a small radio-frequency choke coil satisfactory for use in filtering out of a circuit any radio-frequency currents. Three of these chokes were tested in the Laboratory and gave an average inductance of 90 millihenrys and an average resistance of 220 ohms. The impedance of these chokes to the lowest radio frequency used by any broadcasting station is approximately 283,000 ohms. Their impedance at the highest audio frequencies, say 8000 cycles, is only 4500 ohms, approximately, which is negligibly small in comparison with the impedance of an audio transformer. Two of these choke coils were used in the two-tube shielded "Lab" receiver which was designed and constructed by Mr. John B. Brennan, one of them being located in the radio-frequency stage and the other in the plate circuit of the detector tube. These r.f. chokes measure about 1 \" long and are about the same height. They are supplied with both soldering lugs and binding posts. Price $2.00. Type 125 — This choke coil is similar to the type 85 and can be used for the same purposes. Three of these chokes gave an average inductance of 200 millihenrys and an average d.c. resistance of 400 ohms when measured in the Laboratory. Homeconstructors unfamiliar with chokes and their uses might consider a resistance of 400 ohms to be too high, but this is not so. Suppose we had a 20 1 -a tube in an r.f. amplifier, operating with 90 volts on the plate and a 45-voIt C battery. The plate current would be about 0.002 amperes. Now, if we wanted to keep the r.f. currents out of the B battery we would put a choke coil, Ki in Fig. 1, in the plate circuit of the tube, and then bypass the r.f. currents to the filament with the condenser O. If the choke coil was a type 125 its impedance at 500 kilocycles would be 627,000 ohms. Its impedance, being proportional to the frequency, would be even higher at higher frequencies. The drop in plate voltage across this choke coil will be equal to the plate current (0.002 amperes) times the d.c. resistance (400 ohms). The loss in plate voltage then figures to be 0.8 volts, which is an insignificant amount. This is the only effect of the choke coils resistance (not impedance), that is, to cause a loss in plate voltage, and an ordinary resistance could be used with equally effective results were it not for the comparatively high loss in plate voltage which it would cause. If a resistance were to be used, it would be necessary to employ one with a value of 627,000 ohms, and then the loss in plate voltagewould be 627,000 x .002 = 1254 volts. To impress 90 volts on the plate would require a battery with a voltage of 1254 + 90, or 1344 volts! Hence a resistance is entirely unsuited. If you ever want to arrange a receiver so that the radio frequency currents in the plate circuit are kept out of the battery, use a choke coil and never a resistance. Price $2.25. Type 3 — This choke coil is for use in filtering audio frequency currents out of any circuit. The average inductance of three samples proved to be 3.1 henrys, and the average resistance, 586 ohms. These choke coils are suitable for use in audio frequency circuits whereas the other choke coils are designed for radio-frequency circuits. Two points at which these chokes might be used are indicated in Fig. I, K2, where they function to prevent, to a considerable extent, coupling in the plate supply which might cause howling. These chokes are usually unnecessary when the amplifier is supplied from B batteries but are frequently of value if a line power-supply device is used for the plate supply. They measure about 4" long. Price $3.25. Type D — This is a dual impedance made in accordance with the Donle design. Three of them can be used in constructing a three-stage high-quality audio amplifier. Price $5.00. Type G — This is a grid impedance satisfactory for use in the grid circuits of an impedancecoupled amplifier. Price $4.50. Type P — A plate impedance of this type may be used in the plate circuit of an impedance amplifier, and an excellent double impedance amplifier may be made up using type P impedances in the plate circuits and type G impedances in the grid circuits. Price $4.50. Type O — This is an output impedance to be used in eliminating the direct current from the windings of a loud speaker. A fixed condenser with a capacity of two microfarads or more must be used in conjunction with this output device. See Fig. I. Note how the audio currents indicated by the dotted lines flow directly back to the filament and do not pass through the battery. Price $5.00. Symphonic Transformers — The Symphonic audio frequency transformer is a comparatively new product of the Samson Company put on the market to supply the homeconstructors' demand for a very high-quality unit. The transformer is enclosed in a brushed copper case which gives it a very nice appearance. Two of these units were used in an amplifier constructed in the Laboratory, and proved very satisfactory. The amplifier which was constructed is illustrated on page 494 and it was used with good results in conjunction with several different receivers, including the R. B. "Lab" two-tube shielded receiver. The price of the Symphonic Transformers is $9.00 each. A GROUP OF SAMSON APPARATUS The No. 3 choke is in the foreground and the small choke in the rear is a type 125. The grid impedance, type G, is on the left. The other impedances made by Samson are similar in appearance In connection with the use of the above-mentioned Samson apparatus we would suggest that interested home-constructors write to the Samson Electric Manufacturing Company and obtain from them their booklet entitled Audio Amplification. This booklet, costing twenty-five cents, contains a great deal of information regarding circuits using the apparatus described here. TTHI 1 na POWER-SUPPLY DEVICE IE Greene-Brown Company's B power-supply device is an excellent unit capable of supplying sufficient voltage for the operation of an ordinary radio receiver using a 171 type tube in the output. A photograph of this plate supply unit is shown on page 494. It is equipped with a high